OK, let's calm down, this needs a bit of explanation before proceeding.

ISP's usually block port 25 :
Unlike many ISP's, mine doesn't ! They still allow customers to send emails directly through and to any SMTP servers (tcp/25).

The goal in blocking port 25 is to block viruses from spreading around by sending emails using their own SMTP daemon.
At work, by just reading our email server logs, I know which ISP's aren't blocking port TCP/25 (damn Wanadoo and Road Runner).

If your ISP blocks port TCP/25, you need to send emails through their own (usually overwhelmed) SMTP server.

Worst case scenario :
Let's say someone breaks into my wireless network with a linux laptop (pretty unlikely with WPA2 security but who knows :) ), the attacker would be able to send as much spam as one would like using a local sendmail or postfix server.

To fill that breach, we need to block port tcp/25 for wireless clients.

For MAC filtering to be really useful, you should run arpwatch along which would send you an alert if the system finds out weird activity going on with MAC addresses : http://ee.lbl.gov/

As a security measure, you would also prohibit access to the linux gateway SMTP server (and trying to set it up properly as well) :
iptables -A INPUT -i eth2 -p tcp -m tcp --dport 25 -j DROP

Blocking port 25 data forward under a wired business network could be useful, it could block potential viruses on client machines (who said PC's running Windows ?) from sending emails out, you can even log the illegal activity on port 25 so you can detect any virus presence on the network.

As a security measure, please consider switching your wireless network to WPA or WPA2.
Indeed, it is so easy to crack a WEP network and circumvent MAC filters and absence of a DHCP server.

Setting up a transparent web proxy can also help finding out weird activity coming from computers on the network. Some viruses attempt to download stuff from the web at specific time of the day. That's how I noticed one computer on a network was always trying to access some weird site, every night at 3am.